1. Field of the Invention
This invention relates to an image-forming apparatus and an image-forming method which make use of an amorphous-silicon electrophotographic photosensitive member, a contact charging means and a spherical toner.
2. Related Background Art
As techniques for element members used in electrophotographic photosensitive members, proposals are made on various materials such as selenium, cadmium sulfide, zinc oxide, phthalocyanine and amorphous silicon (hereinafter xe2x80x9ca-Sixe2x80x9d). In particular, non-single-crystal deposited films containing silicon atoms as a chief component as typified by a-Si films and also amorphous deposited films formed of a-Si compensated with. e.g., hydrogen and/or a halogen (such as fluorine or chlorine) have been proposed for high-performance, high-durability and environmental-pollution-free photosensitive members, and some of them have been put into practical use.
In recent years, under circumstances where electrophotographic apparatus are demanded to be made more and more high-performance, electrophotographic apparatus making use of a-Si are also demanded to achieve higher image quality and higher resolution than ever.
Conventionally, it is common to use corona charging assemblies in charging units for photosensitive members used in, e.g., plain-paper copying machines, laser beam printers, LED printers and liquid-crystal shutter printers, and such corona charging assemblies are in wide use. The corona charging assemblies charge object members electrostatically by applying a high voltage of about 5 to 10 kV to a metal wire of about 50 to 100 xcexcm in diameter to ionize the atmosphere.
For structural reasons, the corona charging assemblies have a disadvantage that generation of ozone in a large quantity accompanies corona discharging. With their repeated used, ozone and corona products may become deposited on the photosensitive member surface, under the influence of which the photosensitive member surface may become susceptible to humidity to tend to absorb moisture content. This may cause a lateral flow of electric charges on the photosensitive member surface in an environment of high temperature and high humidity to cause a lowering of image quality which is called smeared images. There is such a problem. In particular, the electrophotographic photosensitive members making use of a-Si have so high a surface hardness that. while they are durable to printing on a large number of sheets, their surfaces may abrade with difficulty. Hence, corona products having once adhered can be removed with difficulty to have a great influence.
As another problem of corona charging assemblies, they tends to be affected by any uneven layer thickness and resistance distribution of the photosensitive member. This may cause unevenness in surface potential, and may consequently cause uneven density on images.
In order to solve such a problem on image quality, various charging units are proposed.
In a contact charging unit as disclosed in Japanese Patent Application Laid-open No. 63-208878, a charging member to which a voltage is kept applied is brought into contact with an object member to be charged (photosensitive member), which is called charging object member, to charge the photosensitive member surface to an intended potential. Compared with the corona charging assemblies, such a unit can achieve a low voltage in respect of the applied voltage necessary for providing the desired potential on the charging object member surface, and does not cause any smeared images due to the ozone products because the quantity of ozone occurring in the course of charging is zero or is very small. Also, in such contact charging, the surface of the photosensitive member is charged to have substantially a uniform potential in accordance with the applied voltage, and hence uneven image density may little occur. It has such advantages.
In the way of progress such that a series of contact charging members are improved in various manners. as disclosed in Japanese Patent Application Laid-open No. 8-6353, a mechanism is proposed in which a contact charging member making use of particles in the form of a magnetic brush comprised of a magnetic material and magnetic particles (or powder) is brought into contact with an electrophotographic photosensitive member to provide it with charge. Also proposed is, as disclosed in Japanese Patent Application Laid-open No. 10-307454, a new method of a mechanism in which a carrying member having conductivity and elasticity so constructed that charged particles are carried on the surface is brought into contact with a photosensitive member to provide it with charge.
Attempts to achieve much higher image quality are also made from improvements of toners. More specifically, polymerization toners are on studies in place of conventional pulverization toners.
The polymerization toners have superior fluidity because they have particles in substantially a uniform spherical shape and having less scattering in particle diameter, Also, they are advantageous to the achievement of high image quality because they do not let colorants come bare to particle surfaces and have uniform triboelectric chargeability. Still also, they can enclose wax in particles, and can attain good fixing performance and anti-offset properties. Hence, the polymerization toners are being gradually widely employed in high-image-quality machines. As a patent application which proposes a magnetic polymerization toner, EP1058157 A1 is accessible.
As stated above, attempts to achieve much higher image quality are being made by combining the formation of uniform latent images free of any unfocused or uneven images that is attributable to contact charging units with the formation of faithful visible images that is attributable to polymerization toners.
However, in the case of high-image-quality image-forming apparatus in which uniform latent images are formed by utilizing the voltage application type contact charging unit as a means for charging the electrophotographic photosensitive member and the latent images are rendered visible by the use of the polymerization toner that can perform highly precise development as stated above, there are the following problems.
That is, in such a contact charging unit, it has very good charge potential uniformity when viewed microscopically as stated above. However, when viewed microscopically, for the reasons of its construction, marks of contact of the magnetic brush or charged particles with the photosensitive member (brush images) may appear. Such brush images appear at halftone image areas. and hence it makes image quality very poor even when the polymerization toner is used.
In order to prevent such brush images, it is necessary to make higher the relative speed between the charging unit and the photosensitive member to make them rub against each other in a greater extent so that the charging unit can be brought into uniform contact with the electrophotographic photosensitive member. However, because of such rubbing, the surface of the photosensitive member may abrade or wear, though slightly. Although such wear is at a small level, even microscopic abrasion may have a great influence when it lasts over a long period time, because the a-Si photosensitive member has a long lifetime originally.
As another problem other than such uneven charging, there is also a problem that the contact charging units deteriorate. For example, in the case of a magnetic-brush type contact charging assembly, its magnetic particles may migrate to the electrophotographic photosensitive member side, which is a problem of what is called magnetic-particle leakage. In the case of an elastic-roller type contact charging assembly, there is a problem that its roller may wear or deform. Once the contact charging unit has deteriorated in this way, faulty charging may occur or image deterioration may occur. Hence, this provides a subject on how the contact charging units be made to have long lifetime.
Many proposals are also made on the improvement of photosensitive members themselves. As a patent application concerning an a-Si photosensitive member having a surface layer formed of a non-single-crystal carbon film, Japanese Patent Application Laid-open No. 11-184121 is accessible.
An object of the present invention is to provide an image-forming apparatus and an image-forming method which have solved the above problems.
Stated more specifically, an object of the present invention is to provide an image-forming apparatus and an image-forming method which make use of an a-Si photosensitive member and promise a high image quality, having been improved in contact performance although the abrasion level of the photosensitive member surface is reduced.
Another object of the present invention is to provide an image-forming apparatus and an image-forming method which are able to obtain high-quality images free of any unfocused images and smeared images in every environment, without causing any generation of ozone products due to corona discharging.
Still another object of the present invention is to provide an image-forming apparatus and an image-forming method in which the a-Si photosensitive member can uniformly be charged to obtain uniform images free of any uneven images and also free of any brush images or coarse images in halftone images.
A further object of the present invention is to provide an image-forming apparatus and an image-forming method which are able to obtain sharp images in a high resolution.
A still further object of the present invention is to provide an image-forming apparatus and an image-forming method in which the a-Si photosensitive member does not wear and operates stably over a long period of time.
A still further object of the present invention is to provide an image-forming apparatus and an image-forming method in which the contact charging unit has a long lifetime and images can stably be obtained at a low maintenance cost and over a long period of time.
The present invention provides an image-forming apparatus comprising:
an electrophotographic photosensitive member having a conductive substrate, and at least a photoconductive layer and a surface layer on the conductive substrate;
a charging means for charging the electrophotographic photosensitive member electrostatically;
a latent-image-forming means for performing imagewise exposure to form an electrostatic latent image on the electrophotographic photosensitive member;
a developing means for moving a toner to the electrostatic latent image formed on the electrophotographic photosensitive member, to render the electrostatic latent image visible to form a toner image; and
a transfer means for transferring the toner image to a transfer medium;
wherein;
the photoconductive layer of the electrophotographic photosensitive member comprises a non-single-crystal material composed chiefly of silicon;
the surface layer of the electrophotographic photosensitive member comprises a non-single-crystal carbon film containing at least hydrogen, and has a dynamic hardness in the range of from 4.90xc3x97109 to 1.76xc3x971010 Pa (500 to 1,800 kgf/mm2);
the charging means has a charging member kept in contact with the electrophotographic photosensitive member, forming a contact zone therewith, and is a charging means for charging the electrophotographic photosensitive member electrostatically upon application of a voltage; and
the toner is a magnetic toner having toner particles containing at least a binder resin and a magnetic material, and an inorganic fine powder, and having an average circularity of from 0.950 to 1.000;
the toner having a saturation magnetization of from 10 to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds).
The present invention also provides an image-forming method comprising:
a charging step of electrostatically charging an electrophotographic photosensitive member having a conductive substrate, and at least a photoconductive layer and a surface layer on the conductive substrate;
a latent-image-forming step of forming an electrostatic latent image on the electrophotographic photosensitive member by imagewise exposure;
a developing step of moving a toner to the electrostatic latent image formed on the electrophotographic photosensitive member, to render the electrostatic latent image visible to form a toner image; and
a transfer step of transferring the toner image to a transfer medium;
wherein;
the photoconductive layer of the electrophotographic photosensitive member comprises a non-single-crystal material composed chiefly of silicon;
the surface layer of the electrophotographic photosensitive member comprises a non-single-crystal carbon film containing at least hydrogen, and has a dynamic hardness in the range of from 4.90xc3x97109 to 1.76xc3x971010 Pa (500 to 1,800 kgf/mm2);
the charging step is a charging step of applying a voltage to a charging member kept in contact with the electrophotographic photosensitive member, forming a contact zone therewith, to charge the electrophotographic photosensitive member electrostatically; and
the toner is a magnetic toner having toner particles containing at least a binder resin and a magnetic material, and an inorganic fine powder, and having an average circularity of from 0.950 to 1.000;
the toner having a saturation magnetization of from 10 to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds).